Motor vehicles emit hydrocarbons as a result of the evaporation of fuel. Generally, such evaporative emissions result from the venting of fuel vapors from the fuel tank due to diurnal changes in ambient pressure and/or temperature, the vaporization of fuel by a hot engine and/or exhaust system, and the escape of fuel vapors during refueling of the vehicle. The venting of fuel vapor from the fuel tank due to diurnal pressure and/or temperature changes (i.e., diurnal emissions) is responsible for a majority of evaporative emissions. Diurnal changes in pressure and/or temperature cause air to flow into and out of the fuel tank. Air flowing out of the fuel tank inevitably carries fuel vapor, which is created by the evaporation of fuel into the air contained above the fuel within the fuel tank. If this flow of air is left untreated and is allowed to escape directly into the atmosphere, undesirable emissions occur.
Motor vehicle manufacturers have reduced the level of diurnal emissions through the use of evaporative canisters. Generally, an evaporative canister has a vapor inlet, a purge port, and a vent port. The vapor inlet is fluidly connected by a vapor conduit to the air space in the fuel tank. Diurnal changes in pressure and/or temperature cause air within the fuel tank to flow through the vapor conduit and into the evaporative canister via the vapor inlet. The air carries fuel vapor and/or hydrocarbons. The evaporative canister contains a sorbent material, such as an activated carbon, that strips fuel vapor from the air as it flows through the canister. The treated air then flows out the vent port and into the atmosphere. The purge port is fluidly connected by a valved purge conduit to the combustion air intake of the motor vehicle engine. When the engine is running, the combustion air intake is at sub-atmospheric pressure, and the valve is opened to thereby connect the purge port to the combustion air intake. Fresh air is drawn by the sub-atmospheric pressure through the vent port and into the evaporative canister. The fresh air flows through the sorbent material, out the purge port and into the combustion air inlet. The flow of fresh air through the evaporative canister strips sorbent material of stored fuel vapor and/or hydrocarbons, thereby purging the evaporative canister of hydrocarbons.
Due to incomplete desorption of the hydrocarbons, minute levels of hydrocarbons remain stored in the sorbent material of a purged evaporative canister. Bleed emissions are believed to result from the release of these stored hydrocarbons (i.e., the hydrocarbon heel) from the evaporative canister into the atmosphere. The term “heel” as used herein refers to residual hydrocarbons generally present on a sorbent material when the canister is in a purged or “clean” state. Bleed emissions may occur, for example, during the heating of the fuel tank during a diurnal cycle. The heating of the fuel tank causes air to flow from the fuel tank, through the canister, out the vent port and into the atmosphere. The air carries the hydrocarbon heel out of the canister and into the atmosphere, thereby resulting in the release of bleed emissions.
Additionally, due to increased legislation on emission standards, vehicles are being mandated to have “zero” evaporative emissions due to gasoline vapors. These types of vehicles may be referred to as a Super Ultra Low Emissions Vehicle (SULEV) or as a Practically Zero Emissions Vehicle (PZEV). To meet the new emission standards, an additional adsorbing media, (e.g., a hydrocarbon scrubber) has been added to the air inlet side of the evaporative canister to capture bleed emissions.
Generally, hydrocarbon scrubbers are constructed using activated carbon powder in a ceramic substrate. The hydrocarbon scrubber may be remotely connected to the evaporative canister's air inlet or integrated into the canister. The ceramic substrate honeycomb also employs seals to prevent hydrocarbon flow around the honeycomb and to protect it from breakage due to the brittle nature of the ceramic.
Therefore, what is needed in the art is an improved hydrocarbon scrubber.